Experimental setup and design.

<p>(a) Schema of setup. A camera tracked the position of a test rat in the arena, where a square pillar stood at the center. The arena was virtually divided into eight subfields, to each of which a different auditory environment was assigned. One of these tones was presented depending on the location of the gravity center of a test rat shown by a red dot. (b) Daily protocol. A habituation period was given first. A single session of sound preference test lasted 20 s, and 25 trials were repeated daily. Each session had a different spatial assignment of tone frequency. Each session was interleaved by a 5-s white noise period. (c) Schedule of the experiments. Preference was measured daily. Day 0 was defined as the first day of conditioning, which associated a 20-kHz tone with rewarding microstimulation of the dopaminergic system. Innate preference was characterized on day 0. A preference test on day 1 or later examined whether and how the conditioning modified the innate preference.